ABSTRACT: -A ~1200-year record of hydrologic variability in the Slave River system.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

The Slave River Delta, NWT, represents a pivotal node in the upper Mackenzie Basin watershed and is a productive northern wetland ecosystem with a rich natural and cultural heritage. Concerns over environmental consequences of natural and anthropogenic-driven decline in river discharge as well as climate variability have prompted hydroecological studies to improve understanding of how this ecosystem functions over time and space. Here we provide results from ongoing multi-proxy paleolimnological analyses that reveal substantialhydrologic variability in the Slave River system. Study sites include two small closed-drainage lakes (GSL1, SD34) located near the margin of a former Great Slave Lake high-stand and another lake (SR1) located upstream on an island in the Slave River. Preliminary development of the chronology of an 80-cm core from SD34 indicates the record spans ~1200 years. Loss on- ignition analyses, as well as organic carbon and nitrogen elemental and isotopic measurements on the sediment core from SD34 indicate that the d13C record provides the most sensitive record of hydrologic variability. The SD34 d13C record identifies four zones, two with high values (Zone 1: ~780 to ~980 AD and Zone 3: ~1240 to ~1800 AD) and two with low values (Zone 2: ~980 to ~1240 AD and Zone 4: ~1800 to ~1950 AD). Isotopic studies of dissolved inorganic carbon (DIC) in the upstream Peace-Athabasca Delta show that the DIC d13C values in closed-basins are low, whereas rivers and large open-drainage lakes have DIC d13C values that are high, indicating strong sensitivity to hydrologic setting (Lyons, MSc thesis in progress). On this basis, the high d13C values of Zone 1 may be due to inundation of the basin from a high-stand of Great Slave Lake and is consistent with silty clay material observed in this zone. The termination of Zone 1 and subsequent lowering of d13C values in Zone 2 is associated with an increase in organic content. These results suggest isolation of SD34 occurred as a result of lower Great Slave Lake water level. The onset of Zone 3 is marked by an abrupt increase in d13C values indicating a rise in Great Slave Lake water levels that inundated SD34. Zone 4 is characterized by a steady decline in d13C values that continues to the top of the core, suggesting a second lowering of Great Slave Lake and subsequent reisolation of SD34. This most recent change in Great Slave Lake water level corresponds with stratigraphic evidence from SR1 of declining Slave River discharge after ~1800 AD. The paleohydrologic record at SD34 broadly corresponds with a millennial water level record for Lake Athabasca suggesting that discharge in the Peace-Athabasca-Slave river system hasvaried considerably over the past ~1200 years. Furthermore, these results show that water levels in the upper Mackenzie River system have been in decline over the last 200 years – a decline that is likely to continue given expected trajectories in river discharge.

Bibliography of Canadian Geomorphology